The unexpected death of Supreme Court Justice Antonin Scalia on Saturday raises the curtain on what promises to be a bitter confirmation battle, which in turn will dramatize some important truths about the presidential election campaign.

It's been almost three decades since Ronald Reagan departed the White House. Yet his Supreme Court nominations still detonate explosive controversies.

The sardonic and influential Scalia was in his 30th term on the high court. Yet he wasn't the most consequential of Reagan's choices.

Justice Anthony Kennedy, Reagan's final high-court nominee, has long been a crucial swing vote, determining the outcome of momentous cases concerning abortion, gun control, property rights and much more.

Kennedy was the decisive vote — authoring the majority opinions — in both the 2010 Citizens United campaign-finance case (which progressives believe wrecked American democracy) and last year's Obergefell vs. Hodges legalizing same-sex marriage nationwide (which conservatives believe wrecked civilization).

Beyond Reagan's legacies, consider the tenures of Justices Clarence Thomas (nominated by the first President Bush in 1991) and Justices Ruth Bader Ginsburg and Stephen Breyer (Bill Clinton nominees in 1993 and 1994).

Nothing about presidential candidates may matter more in the long run — especially in the long run — than their attitudes toward judicial nominations. In light of Scalia's passing, it's worth noting that Ginsburg, Kennedy and Breyer are all near or beyond 80 years of age. The next president could reshape the court for a generation.

The good news is that this year's presidential hopefuls aren't shy about explaining how they'd choose judges. The bad news is that the approach many proclaim could undermine the integrity of the court.

Bernie Sanders has gone the farthest, recently confirming in debate: "Let me be very clear. No nominee of mine, if I'm elected president, to the United States Supreme Court, will get that nomination unless he or she is loud and clear, and says they will vote to overturn Citizens United."

A so-called "litmus test" can't get more explicit than that — no nomination without a specific pledge to overrule the Supreme Court's controversial holding that the First Amendment protects independent political spending, even by corporations and unions.

Traditionally, presidential candidates have promised not to demand rigid commitments from judges, respecting the ideal that a judge approaches every case with an open and independent mind.

But much is changing in 2016. Sanders' opponent, Hillary Clinton, told a town hall audience "I have a bunch of litmus tests. … We have to preserve marriage equality. … We've got to make sure to preserve Roe vs. Wade …"

This isn't as clear as Sanders' "no … nomination unless" formulation. Much depends on what the exact Clintonesque meaning of "litmus test" is. But it's too clear.

Displeasure with the Supreme Court is one sentiment that unites Democrats and Republicans, although they're overwrought for opposite reasons. Yet as a group, remaining Republican candidates have been more circumspect about demanding specific pledges from judicial appointees, despite conservative bitterness over last summer's marriage ruling and the court's upholding of Obamacare, and, of course, despite the unending loathing of Roe. vs. Wade.

Still, some go beyond what used to be considered seemly.

Marco Rubio told one interviewer: "[T]here's no way that you can read that Constitution and deduce … a constitutional right to an abortion, or … to marry someone of the same sex. … So you need judges that understand how constitutionally flawed those two kinds of rulings and others have been …"

Donald Trump has been characteristically odd and vague about the court, noting that his sister is a "phenomenal"appeals court judge. John Kasich, Jeb Bush and Ben Carson have generally voiced the traditional reluctance to embrace litmus tests, while of course pledging to nominate only solid conservatives.

Essentially, that's what Ted Cruz says, too, although no candidate in either party conveys deeper distaste for the court's misdeeds. Cruz has various ideas for taming what he sees as a "lawless" court, including amending the Constitution to subject Supreme Court justices to elections. But what he says of nominations is abstracted.

"Every justice I put on that court," Cruz has often repeated, "will be a principled constitutionalist jurist with a proven record who will be faithful to the law and will not legislate from the bench."

Cruz combines this conventional rhetoric with his signature air of angry militance, accusing the rest of the GOP of never having cared enough about the court and of long doing a miserable job of putting true, determined conservatives there — while Democratic presidents' nominees have turned out to be reliably liberal jurists.

Cruz has a point. Justice Kennedy is a prime example — a judge chosen by a conservative president who has disappointed the right on big cases. Justice Sandra Day O'Connor was another, along with Justice David Souter, nominated by the first President Bush. Even Chief Justice John Roberts, nominated by George W. Bush, has twice rescued Obamacare.

It is hard to find modern justices named by Democratic presidents who have proved equally disappointing to liberals.

An older style of conservatism might have accepted more serenely that "conservative" judges, faithful to the rule of law, would naturally prove less predictable and ideological than liberal "activists." But much has changed in 2016.

Prudent Americans across the political spectrum should notice that critical boundaries are being strained this year. Of course, all presidents have sought to name like-minded judges who would make congenial rulings. Some have improperly muscled the court (Franklin Roosevelt's "court-packing" scheme comes to mind). And of course, neutrality and open-mindedness can never be other than ideals imperfectly aspired to.

But to openly turn judging into political falconry, in which justices purchase their seats on the bench by pledging "loud and clear" not to be impartial and independent on specific issues, is to threaten public confidence that we live under a legitimate government of laws.

The question is whether the U.S. Senate should ever confirm a judge appointed by a politician who proposes extracting such promises.

D.J. Tice is at Doug.Tice@startribune.com.